The isolation platform: This is what I'm using for isolation. It's a granite surface plate from Enco. It weighs 74#, and honestly it looks pretty good! It was $57.88 delivered. Including $35 shipping from Nevada to a suburb of Chicago. It is polished to a satin finish on 5 sides, and unpolished on the bottom. Are the tolerances good enough for our use? 1/10,000 of an inch all the way around.

The supports: I have the slab supported on the 4 corners, with 6 pieces each of 1/5 inch (= about 5 mm) of #30 sorbothane under it, which I bought from E-bay. As I understand it, the next most compliant sorbothane has the compliance of chewing gum. The total height, off of the table that the whole thing sits on, to the bottom of the granite is 1-1/16" (=about 27 MM.) In other words that is the height of the sorbothane after the compression weight of the turntable, and granite.

Sorbothane: For our uses I LOVE this stuff. One thing happened that I didn't expect: I thought the whole unit would have a 3-6 HZ resonance, or springy effect when tapped. It didn't. This is totally unlike using rubber, or springs. When you hit the slab, it barely moves, and has an extremely damped quality to it. It acts like a 'Vette, when you push down on the front, it goes down, pops up to the level it was in and stops dead. (I don't know how else to describe it.) It's like a very low resonance, with a strong shock absorber attached.

Between the granite and sorbothane, I couldn't ask for better results. All acoustic feedback has stopped DEAD. The two non-scientific experiments I did were as follows; In a couple other rigs that I have tried, I always tap hard on the main support table, with my right knuckles, while lightly holding my left finger tips on the turntable support. (In this case, left fingers are on the granite.) With every other rig, I always felt something on the support piece. In this case I couldn't feel anything with my left fingertips on the granite.

The other semi-scientific test, was to keep the turntable turned off, (!!) place the stylus down, on the record, and turn the volume up all of the way on the rig. [The way it is wired up, is the phono in to the preamp, then the preamp recording outs in to an Outlaw audio 990 Processor. The Outlaw is turned all of the way up] I then carefully, tap around on the support table, the granite, and even the turntable to try and induce feedback. I would here it for a split second, and then it would decay to nothing. In every other setup, it would of course start to howel, and I would have to turn it down. Keep in mind that this is a brutal test, and if I tried to play it at that volume, the two Threshold S-300's would be too busy blowing up the whole system for me to worry about feedback. I am also using two home brew, 6.5 cu.' (184 liter net.) sealed sono subs with a total of 4 peerless 260-SWR's. In my 22 x 12.5 x 7 ft. room, they go down flat to 15 Hz.
Also, as an added, unexpected bonus, when you walk around the area of the turntable while it plays, it will not skip or wobble like I thought it would. I think that is because of the damped nature of the combination.
Anyway, two downsides were a chip in the rear corner, (which I'm sure they would have replaced) and, believe it or not, the granite rings just a bit when you tap on it. I think what I will wind up doing is attaching a corian slab on top, because as I knew, it's not quite big enough for the dustcover to rest properly over the turntable, but the cover will work on it for now.

(pic will follow shortly) The pic is a shot of my RM-9 with a Bluepoint Evo-3, and stock arm on it. I got that on E-bay with 50 hrs on it for $875 delivered. (Believe me after reading the whole thread on valveitude's (Casey's) gargantuan effort, I now want to build my own table) I'm using a Spectral DMC-6 preamp which was $2,850 originally. I bought it about 10 years ago for $1,350, and now they are going from $650-$750 on E-bay and Stereophile sites. The phono section of this preamp is by far, the quietest I've ever owned.

Anyway, I hope this idea is as helpful as it is simple for other people and their turntable isolation problems.

Since this thread never got any replies, I don't know if anyone even cares, but here goes!

So I glued the 1/5" thick sorbothane to the entire bottom of the plate, and believe it or not it killed off about 95% of the little bit of ring that the slab had. The thing is about sorbothane, which is now one of my favorite materials for our kind of stuff, is it really is now pretty expensive. I'm thinking the gas crunch might have helped that along, but I found out the stuff is actually made from polyurethane.
The cost of the plate was under $30, shipping was $35, and if I didn't get a great deal on the sorbo from Ebay, that would have been $45. It can be bought at Macmaster Carr. I'm thinking also that two granite slabs, with sorbo laminated in the middle, might make one hell of a turntable plinth.

I love sorbothane too. I have a few different types. I have some large semi-hemispherical shapes under my CD player but, for some reason, they leak brown goo!! None of the other stuff does though.
I have found that it's also quite easy to 'kill' sorbothane due to over compression. I suppose you have to get the pressure just right.